Fairhaven's sewage system, digester aren't getting along

FAIRHAVEN — The wastewater treatment plant's new anaerobic digester has continued to trouble the Board of Public Works since it became operational in September.

ARIEL WITTENBERG

FAIRHAVEN — The wastewater treatment plant's new anaerobic digester has continued to trouble the Board of Public Works since it became operational in September.

After months of mechanical issues, the now functional digester is "not integrating well" with the rest of the treatment system, which was last updated in the 1980s.

"It's like a marriage that needs counseling," BPW Superintendent Vincent Furtado said. "Both people individually are good, but we need to get them to work together."

When wastewater arrives at the treatment plant, it goes into a "settling tank," where it sits to allow the solid waste to be removed by sinking to the bottom.

Without a digester, the solid waste that sinks is trucked off-site for disposal, something the town must pay for by the pound.

Using a digester, that sunken solid waste instead moves to an oxygen-free chamber where it is broken down into a methane-like gas by bacteria. The gas is then sold to energy companies to defray costs.

A digester does not break down all the solid waste at once. "Side streams" of sludge circulate out of the digester and to the beginning of the treatment process, where, Furtado said, the plant is now running into problems.

When wastewater arrives at the plant from the sewers, Furtado said, it is usually around 50 degrees. But the recirculation from the digester is closer to 100 degrees because it has been heated by the methane-like gas created by the bacteria. Furtado said the temperature difference adversely affects the settling process.

"Colder material is denser and settles better," he said. "When you start introducing more hot water the whole process doesn't work as well."

Less solid waste settling means more of it remains in the water when it is discharged into the harbor.

Furtado said there is a worry that if the digester runs at full capacity, it could put the plant close to being in violation of its discharge permit, which regulates how clean the water must be.

"Basically we are trying to balance how we optimize the digester to get the profit we were hoping for but also minimize the effect it has on the other side of the plant," he said.

Since becoming functional in September, Furtado said, the digester has been taken online and offline a number of times to allow staff to "work out the bugs in the system."

"Every time we do something, we have to turn it off and we start back in square one," he said.